Please note that it's not necessary for you to purchase credits before placing a translation order. You can simply check out by using credit cards / PayPal directly at checkout.
The purpose of having credits is to avoid the extra charges by the payment gateways on small orders.
How to purchase credits and check previous credit usage
1. Click on your icon at the top right corner and select "Billing" from the drop down menu
2. Click "Purchase credits" to purchase more credits and see the previous credit usage records